The present invention relates to an apparatus for impregnating reinforcing materials with a resinous binder material. The reinforcing materials may, for example, comprise fiber reinforcing materials such as rovings, fleeces, belts, cords, webbings or the like forming bands or strips. The term "band" or "bands" will be used herein as a generic word for the words listed. These bands of reinforcing materials are impregnated by a resinous binder agent for producing fiber compound materials, for example for producing helicopter rotor blades. Conventionally, the resionous binder material is applied to one surface of the reinforcing material.
Where fiber reinforced compound materials are used in the construction of components which are subject to high dynamic wear and tear, it is necessary not only to exclude air bubbles from the impregnated fiber compound material, but also to make sure that the adhesion of the impregnating resin material on the entire surface of the reinforcing fiber material is complete. Spots of non-adhesion have been found to cause a rapid deterioration of the strength characteristic of helicopter rotor blades.
U.S. Pat. No. 3,762,364 (Funsch et al) discloses the coating of synthetic fiber webbings with an adhesive and with rubber for producing car tires. The adhesive is applied to the advancing fabric sheet or belt as the latter is dipped into an adhesive dipping pan, whereby the fabric web is coated with adhesive on both surfaces. Downstream of the dipping pan there is arranged a set of so-called hold-back rollers followed by a drying zone, a heat zone, and a normalizing zone forming together a hot stretching unit which in turn is followed by puller rollers. Downstream of the puller rollers a fabric flexing unit is arranged which in turn is followed by the calendering unit for coating one surface of the fabric with the rubber. The complete exclusion or avoidance of air inclusions in the interstices between adjacent fiber threads is not essential in the construction of pneumatic tires. Thus, in U.S. Pat. No. 3,762,364 the flexing station actually increases the volume of the adhesive coated fabric because the flexibility is more important than the total avoidance of air inclusions. U.S. Pat. No. 3,762,364 refers to minimizing entrapped air pockets, however, such minimizing relates to air pockets between the metal beads of a tire building drum and the tire material prepared as described above.
In other conventional methods a paper web or a reinforcing material is guided over a rotatable impregnating roller dipping into a bath of the resinous binder material. U.S. Pat. No. 4,241,690 (Muller) is representative of such coating. In such methods it is customary to adjust the ratio of the circumferential speed of the impregnating roller to the feed advance speed of the reinforcing material or paper web in order to vary the degree of application of binder material or other coating material to the reinforcing material or other type of web. Excess resinous material may be stripped off, for example, by means of doctor blades or by means of squeezing rollers, please see also German Patent Publication (DE-AS) No. 2,012,107.
According to the prior art method it is possible to apply the binder material in a single operation to the reinforcing material in a precisely adjustable ratio between the binder material and the reinforcing material. This operation may be accomplished in the prior art method in a continuous manner. Nevertheless, the impregnation achievable according to the prior art is not optimal as far as 100% adhesion is concerned. Defects in the impregnation are especially apparent when rovings prepared according to known methods are used in the so-called thimble or deadeye zone of the main rotor blades of a helicopter. It has been found that the rovings suffer in the blade root zone, which is subject to high loads, a premature shearing failure already at relatively low load cycles. The failure occurs substantially spontaneously within a few thousand load cycles following the first failure indications. The separating break has the appearance of a failure due to tension loads of the glass fibers. However, the failure is actually due to shearing fractures or cracks and is believed to be due to inadequate adhesion between the total available surface of the reinforcing material and the matrix formed by the impregnating resinous material.